Implement till 4.30

ex-4_25-30.scm

@@ -128,15 +128,6 @@
 
 (define the-global-environment (setup-environment))
 
-(eval '(define (assert a b)
-  (cond ((equal? a b) (display "[ok]"))
-    (else
-      (display "[error] ")
-      (display a)
-      (display " != ")
-      (display b)))
-  (newline)) the-global-environment)
-
 (eval '(define (unless condition usual-value exceptional-value)
   (if condition exceptional-value usual-value)) the-global-environment)
 
@@ -145,12 +136,12 @@
           (* n (factorial (- n 1)))
           1)) the-global-environment)
 
-(eval '(assert (factorial 5) 120) the-global-environment)
+(assert (eval-force '(factorial 5) the-global-environment) 120)
 
 (display "\nex-4.27 - lazy-evaluator\n")
 
 (eval '(define count 0) the-global-environment)
-(eval '(define (id x) (set! count (+ count 1)) x) the-global-environment))
+(eval '(define (id x) (set! count (+ count 1)) x) the-global-environment)
 (eval '(define w (id (id 10))) the-global-environment)
 
 ; I expected count to be 0 here, but it is 1 in reality. I think when
@@ -160,12 +151,76 @@
 
 ; w is thunk and cannot be displayed. By getting the actual value it will show
 ; 10 as expected.
-(assert (actual-value 'w the-global-environment) 10)
+(assert (eval-force 'w the-global-environment) 10)
 
 ; After w is fully evaluated count is 2 as expected.
 (assert (eval 'count the-global-environment) 2) ; count is 2
 
-(display "\nex-4.28\n")
+(display "\nex-4.28 - eval-actual-value\n")
 
-(display "\nex-4.29\n")
+; Replace actual-value with eval in line 53 and see how this fails: unknown
+; procedure (thunk + ...).
+(assert (eval '((lambda (op a b) (op a b)) + 1 2) the-global-environment) 3)
+
+(display "\nex-4.29 - memoization\n")
+
+; Calculating fibonacci numbers recursively will run much slow without
+; memoization.
+(define the-global-environment (setup-environment))
+(eval '(define (fib n)
+         (if (< n 2)
+           1
+           (+ (fib (- n 2)) (fib (- n 1)))))
+      the-global-environment)
+(eval '(define count 0) the-global-environment)
+(eval '(define (id x) (set! count (+ count 1)) x) the-global-environment)
+(eval '(define (square x) (* x x)) the-global-environment)
+
+(assert (eval '(square (id 10)) the-global-environment) 100)
+(assert (eval 'count the-global-environment) 1)
+(assert (eval '(fib 10) the-global-environment) 89)
+
+; If we implemented force-it without memoization count would be 2 because the
+; id-procedure gets evaluated twice:
+; (assert (eval 'count the-global-environment) 2)
+
+(display "\nex-4.30 - eval-sequence\n")
+
+; a. Ben is right because primitive procedures are executed right away.
+
+; b.
+
+(eval
+  '(define (p1 x)
+  (set! x (cons x '(2)))
+  x)
+  the-global-environment)
+
+(eval
+  '(define (p2 x)
+    (define (p e)
+      e
+      x)
+  (p (set! x (cons x '(2)))))
+  the-global-environment)
+
+; With the text implementation the result is (1 2) because set! gets evaluated
+; right away.  p2 defines another procedure which has the consequence that e is
+; evaluated lazily. Hence, the intial value of e 1 is returned.
+
+(assert (eval-force '(p1 1) the-global-environment) '(1 2))
+(assert (eval-force '(p2 1) the-global-environment) 1)
+
+; If we change the implementation of eval-sequence the behavior of p1 is the
+; same. However, since e gets evaluated right away in p2 the result is the same
+; as for p1.
+
+; c. Evaluating the statements right away does not change the behavior, because
+; force-it is called either way.
+
+; d. I think the lazy implementation should be kept as it is. Primitive
+; procedures work as expected and if we have a lazy language it is consistent
+; to not force the evaluation of unused thunks.
+
+(display "[done]\n")
 

ex-4_31-34.scm

@@ -0,0 +1,34 @@
+(load "util.scm")
+(load "misc/evaluator.scm")
+
+(display "\nex-4.31 - explicit-laziness\n")
+
+; Exercise 4.31.  The approach taken in this section is somewhat unpleasant,
+; because it makes an incompatible change to Scheme. It might be nicer to
+; implement lazy evaluation as an upward-compatible extension, that is, so that
+; ordinary Scheme programs will work as before. We can do this by extending the
+; syntax of procedure declarations to let the user control whether or not
+; arguments are to be delayed. While we're at it, we may as well also give the
+; user the choice between delaying with and without memoization. For example,
+; the definition
+
+;(define (f a (b lazy) c (d lazy-memo))
+;  ...)
+
+; would define f to be a procedure of four arguments, where the first and third
+; arguments are evaluated when the procedure is called, the second argument is
+; delayed, and the fourth argument is both delayed and memoized. Thus, ordinary
+; procedure definitions will produce the same behavior as ordinary Scheme,
+; while adding the lazy-memo declaration to each parameter of every compound
+; procedure will produce the behavior of the lazy evaluator defined in this
+; section. Design and implement the changes required to produce such an
+; extension to Scheme. You will have to implement new syntax procedures to
+; handle the new syntax for define. You must also arrange for eval or apply to
+; determine when arguments are to be delayed, and to force or delay arguments
+; accordingly, and you must arrange for forcing to memoize or not, as
+; appropriate.
+
+; (display "\nex-4.32\n")
+; (display "\nex-4.33\n")
+; (display "\nex-4.34\n")
+

ex-4_35-xx.scm

@@ -0,0 +1,4 @@
+(load "util.scm")
+
+(display "\nex-4.35\n")
+

misc/evaluator.scm

@@ -305,9 +305,8 @@
                      '<procedure-env>))
       (display object)))
 
-(define (eval-verbose exp env)
-  (let ((output (eval exp env)))
-      (user-print output) (newline)
+(define (eval-force exp env)
+  (let ((output (actual-value exp env)))
       output))